The present invention relates to a passive system and method for measuring the amount of liquid water in the atmosphere and the temperature of the water for a determination whether ice will form on the airframe, and more specifically to a microwave icing avoidance system and method using a passive radiometer utilizing either a single frequency or two frequencies of passive remote sensing microwave energy for monitoring the severity and type of ice in a cloud formation.
Aircraft icing is caused by flight into areas of supercooled cloud water or drizzle, that is, water remaining liquid at temperatures below 0 degrees Celsius (273 K). This is a rather common situation in Fall and Winter, especially at altitudes flown by commuter aircraft and aircraft in holding patterns for an airport. There is no known existing operational passive system or method for recognizing the conditions of supercooled liquid water and/or drizzle in clouds that cause aircraft icing.
U.S. Pat. No. 5,028,929, xe2x80x9cIcing Hazard Detection for Aircraftxe2x80x9d describes a dual-frequency radar system for recognizing the presence of liquid and drizzle droplets in a cloud. However, the radar, which measures energy backscattered from the droplets, cannot estimate droplet temperature. Accordingly, the system described in this U.S. patent cannot tell if water droplets are supercooled (i.e., at a temperature below 273 K) and about to freeze, or whether they are above 273 K and cannot freeze. In addition, the radar system cannot distinguish between liquid water drops, which can cause icing and frozen ice pellets, which are not known to contribute to aircraft icing.
Passive microwave sensors have been used in meteorological satellite programs such as the Defense Meteorological Satellite Program (DMSP) and the Tropical Rainfall Measurement Mission (TRMM) for several years for the study of cloud water content and temperature (Jansen, Michael A., 1993: Atmospheric Remote Sensing by Microwave Radiometry, Wiley Series in Remote Sensing, and Stephens, G. L., 1994: Remote Sensing of the Lower Atmosphere, Oxford Univ. Press). The need for the remote detection of aircraft icing conditions, and the DoD need for covert capability lead to considering this technology for aircraft.
In accordance with the present invention, there is provided a microwave icing avoidance system utilizing a passive microwave radiometer operating at frequencies above and below the 50-60 GHz oxygen absorption band, and observing for cloud droplets or drizzle droplets in the atmosphere ahead of an aircraft or upwind from an airport at three angles of observation: horizontally, and at + and xe2x88x922xc2x0 from the horizontal.
The microwave icing avoidance system (MIA) system of the present invention provides information to a pilot of potential icing conditions ahead of the aircraft (at a distance up to 50 km) or approaching an airport in time for rerouting above, below, or around the danger area. The MIA system also provides the ability to recognize external areas of lesser icing potential from inside an area of icing occurrence. Thus, a pilot who inadvertently enters icing conditions may be able to chart a path out of the area of possible icing conditions.
The use of a passive system is fundamental to the physics of the aircraft icing problem. In addition to detecting the presence of liquid water in the form of cloud droplets or drizzle droplets, an icing avoidance sensor determines whether the cloud droplets are supercooled (that is, at a temperature less than 273 K) or not. Supercooled cloud or drizzle droplets are liable to freeze upon the surface of an aircraft. Droplets above 273 K, on the other hand, usually do not form ice upon the surfaces of an aircraft. A passive icing avoidance system detects the droplets by measurement of emitted radiative energy; such emitted energy is uniquely characteristic of the temperature of the emitter at a given microwave frequency (as given by Planck""s Law).
In addition to being suited to measuring the temperature of liquid droplets in the atmosphere, passive microwave measurement systems are capable of recognizing the difference between liquid droplets and frozen species. This is because the dielectric constant of ice is small, leading to low emissivity by ice. This means that the dangerous supercooled liquid droplets emit most of the radiance sensed by the MIA system radiometer. Since frozen droplets usually do not cause further icing to the surfaces of an aircraft, recognition of the presence of frozen droplets allows the pilot to go safely into the area where such droplets occur. Radar devices, working on the principle of scattering, cannot make this distinction, since ice particles do scatter the radar energy.
The Radiative Transfer Equation (RTE) enables the determination of the brightness temperature coming to the sensor. The brightness temperature depends on the integrated product of the temperature and the derivative of transmissivity along the viewing path. This enables the recognition of the presence of cloud droplets and drizzle droplets by the change caused in transmissivity so long as the viewing path is not horizontal, since the atmosphere tends to be isothermal along a horizontal path. to Accordingly, MIA of the present invention utilizes slightly slanted paths above and below the flight path.
The Mie theory has been utilized in the development of the MIA system of the present invention. According to the Mie theory, the absorption and emission characteristics of a liquid water droplet increases rapidly as the ratio of droplet circumference to wavelength approaches unity. This physical law allows the MIA system to distinguish between ordinary cloud droplets (which are typically about 10 millionths of a meter in diameter) and drizzle droplets (which are typically 350 millionths of a meter in diameter). The MIA system makes use of two different frequencies, such as 37 GHz and 89 GHz, having wavelengths of 8.1 and 3.3 mm respectively. Ordinary cloud droplets have little absorption at both frequencies, since the droplet circumference is small in comparison to the wavelength. However, drizzle droplets have a large absorption and emission at 89 GHz, since the ratio is almost unity.
The MIA system of the present invention utilizes two frequencies to take advantage of Mie theory to xe2x80x9cseexe2x80x9d the droplets in different ways and estimate the water content from the comparison. In addition, the MIA system enables a determination of the distinction between ordinary cloud droplets (usually causing only rime icing) and drizzle droplets, which cause clear icing that may spread beyond the deicing equipment of an aircraft. The tendency of clear ice to freeze slowly and spread beyond ice removal equipment makes this class of ice much more dangerous than rime ice.
Because the MIA system of the present invention is based on the fundamental principle of recognizing the transmissivity decrease caused by liquid water droplets, such a system has additional application. Since droplets reduce transmissivity, rain of intensity greater than drizzle is also very visible in the MIA system, and solid objects such as mountain tops or buildings will also be rendered visible to the pilot within a veil of cloud droplets or haze. Thus the MIA system provides additional flight safety functions related to other weather phenomena and terrain avoidance.
It is also known that a water surface (as distinct from water droplets suspended in air) look much different from land surfaces, whether vegetated, snow covered, or bare soil. The MIA system data, if presented as an image, has utility as a passive, all-weather imager, showing coastlines, water bodies, glaciers, and other natural features, for purposes of navigation. To some extent the MIA system of the present invention performs many functions of a radar, without the cost, power, and overtness of radar transmissions.
The MIA system, in one embodiment, utilizes only one frequency (37 GHz), with a single polarization, and provides information (temperature, liquid water content) sufficient to warn a pilot of icing conditions ahead. For the pilots not qualified to fly into icing conditions, this is all the information needed; they must reroute to avoid the icing conditions. For the smaller but more professional cadre of commercial pilots who are qualified (with appropriate aircraft deicing equipment) to enter icing conditions, an alternate embodiment of the MIA system uses dual frequencies (37 GHz and 89 GHz) and dual polarization at the higher frequency, provides information to enable the distinction between rime ice (which can be removed in flight by deicing boots) and the deadly clear ice. Most commercial flights (and military flights) could proceed with the knowledge that only rime ice is to be expected. This permits optimization of flight operations, rather than unqualified termination for any icing conditions.